1. Field of the Disclosure
The present invention relates generally to power converters. More specifically, examples of the present invention are related to limiting power delivered by a power converter.
2. Background
Many electronic devices include power supplies to provide the devices with regulated direct current (dc) power sources. Switched mode power supplies operating at high frequencies (HF) are very popular due to their small size, good output regulation, high efficiency, and safety features. Switched mode power supplies may be used to convert alternating current (ac) sources or high voltage dc sources into regulated dc sources with desired voltages at the outputs. Depending on the specific applications, different types of switched mode power supplies with different control methods and different features may be utilized.
Typically, a switched mode power supply includes a switching element or power switch controlled by a controller that is coupled to an energy transfer element, such as for example a high frequency transformer or coupled inductor, which provides safety isolation and transforms the voltage level. The output of the transformer is then rectified and filtered to provide a regulated dc output to be provided to an electronic device. The controller may be implemented in an integrated circuit (IC) and the switching element may also be monolithically or non-monolithically included in the same IC. The controller typically receives multiple input signals representative of various parameters of the switch mode power supply. The controller processes the sensed signals and generates control signals to control switching of the power switch between an on state and an off state to regulate the amount of power transferred across the energy transfer element to a load to regulate the output of the power supply in a closed loop.
The output regulation of the power supply is through processing the feedback from the output. The feedback signal from the output can come through an optocoupler from a sense circuit coupled to the dc output. When the feedback is referenced to the secondary ground, it is referred as the secondary control. In some switch mode power supplies, the output sense may be extracted indirectly from a third winding that is magnetically coupled to the secondary winding on the same transformer core. In this example, the feedback signal may be referenced to the primary ground, and is therefore referred to as primary control. The third winding may also provide operating power for controller and is sometimes referred to as a bias or feedback winding. The feedback signal may then be used by the controller to, for example, modulate the pulse width (i.e., PWM), change switching frequency (i.e., PFM), or enable and disable the power switch in some cycle intervals, which is referred as on-off control resulting in controlled pulse skipping in the drive signal used to control switching of the switch.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.